Thermal siphoning in a mass flow controller (MFC) may refer to a continuous circulation of gas caused by the free convection between the heated thermal flow sensor and the bypass. Thermal siphoning may result in a non-zero output signal for the flow rate that resembles zero point drift, even when the actual output flow rate is zero. In some MFC designs, thermal siphoning effects may be more likely to occur if the mass flow controller is installed vertically, and may vary in proportion with the molecular weight and pressure of the fluid whose flow rate is being controlled.
In addition to causing the zero point calibration in mass flow controllers to shift, thermal siphoning may also cause a calibration shift in the span or dynamic range of the mass flow meter of the mass flow controller.
A method and system are needed that can prevent or reduce thermal siphoning effects when a thermal mass flow controller is vertically mounted, and that can provide a zero shift thermal mass flow sensor for a vertical flow thermal mass flow meter.